A cigarette typically consists of a cylindrical tobacco rod, which is wrapped with a cigarette paper, and beyond that optionally contains a filter plug wrapped with a plug wrap paper, which is connected to the tobacco rod by a tipping paper.
It is generally known that the smoker judges a cigarette not only according to its taste during smoking but also by its visual qualities. In particular, a visually homogeneous tobacco rod is considered to be an indication of a high quality cigarette brand, a so called “premium brand”. This can in general mean that the tobacco rod wrapped with the cigarette paper is, visually homogeneous, opaque and white when viewed from the outside, so that tobacco particles of the tobacco rod are not discernible through the paper as a variation in the whiteness.
In order to produce such a visually homogeneous cigarette paper, various means from the prior art are at the disposal of the paper manufacturer. For example, it is known that by increasing the basis weight of the paper, by increasing the quantity of fillers, by the selection of the filler or other measures, the homogeneity of the paper can be improved. All these measures can in fact increase the opacity of the cigarette paper according to ISO 2471 or the whiteness according to ISO 2470-1, but removal of the inhomogeneities is inadequate. It is in the nature of the paper production process that the visual impression of a white paper viewed with backlighting differs substantially from that of a white plastic film by inhomogeneities in the opacity. This overall inhomogeneous visual impression which a paper sheet conveys when viewed under transmitted light is called “formation” or “mottling”. The skilled person typically assesses the formation of the paper subjectively; if the paper sheet is optically homogeneous, its formation is said to be good.
In some cases, such inhomogeneity is desired to a small extent in order to provide the paper with a naturalistic effect and, for example, to make it distinguishable from a plastic film. In other applications, such as on the cigarette, a high visual homogeneity is valued. In Asiatic regions especially, for example, the smoker does not want to be able to discern the tobacco as variations in brightness at some positions through the cigarette paper. This effect is more pronounced the thinner and lighter the cigarette paper and the lower the filler content.
An important visual property of the cigarette paper is its opacity, that is, the opaqueness of the cigarette paper. It is determined according to ISO 2471 and is expressed as percentage from 0% (transparent) to 100% (completely opaque).
Typical cigarette papers are pale grey to white, although black cigarette papers and coloured cigarette papers are also available on the market. The whiteness of the cigarette paper is therefore also an important visual characteristic and is determined according to ISO 2470-1. It is also quantified as a percentage with a value of 0% (black) to 100% (white) in comparison to a white reference material. Values above 100% can be obtained using fluorescence; they are, however, only slightly over 100% and can mostly only be obtained in connection with optical brighteners.
In addition to the visual properties of a cigarette paper, those technical properties of the cigarette paper which can influence the components of the smoke of a cigarette manufactured therefrom play a role. These components are, for example, determined according to a method described in ISO 4387 and comprise, among others, the nicotine-free dry condensate (“tar”), the nicotine content and the quantity of carbon monoxide in the smoke of a cigarette.
An important such property of the cigarette paper is its air permeability. The air permeability is determined according to ISO 2965 and specifies which volume of air flows through the cigarette paper per unit time, per unit area and per pressure difference, and thus it has the dimension cm3/(min cm2 kPa). It is often designated as the CORESTA Unit (CU), (1 CU=1 cm3/(min cm2 kPa)). The air permeability determines, among others, how strongly the smoke is diluted during a puff by the air flowing through the cigarette paper into the tobacco rod.
Another important technical property is the diffusion capacity. It specifies the gas volume passing through the paper per unit time, per unit area and per concentration difference and thus it has the dimension cm3/(s cm2)=cm/s. The diffusion capacity of a cigarette paper for CO2 can, for example, be determined with the CO2 Diffusivity Meter from the company Sodim. The diffusion capacity determines, among others, the gas exchange through the cigarette paper between the tobacco rod and the environment by diffusion, while the cigarette is smouldering.
Treating sections of the cigarette paper with burn-retardant materials to thereby provide the cigarette with self-extinguishing properties is also known in the prior art. A test for determining the self-extinguishing properties is described in ISO 12863. This or very similar tests are also the subject of legal regulations in the USA, Canada, Australia and the European Union. The treated sections are frequently 5 mm to 7 mm wide bands, which are applied to the inside of the cigarette paper and extend in the circumferential direction on the cigarette. The bands obstruct the access of oxygen to the glowing cone of the smouldering cigarette and thus cause self-extinguishing. These strips are usually printed on the wire side of the cigarette, which is generally less suited to printing, instead of on the upper side, so that the printed side is facing the tobacco rod on the cigarette and the bands are less visible from the outside. Despite this, the bands are often detectable on the cigarette with the naked eye upon careful inspection. Together with the inevitable visual inhomogeneity of the paper due to the paper production, such bands formed from burn-retardant material also deteriorate the visual properties.
It has turned out to be difficult to remove said inhomogeneities in the visual appearance. An increase in the basis weight of the cigarette paper is limited by the smoker's acceptance of a cigarette manufactured from this paper, as the smoker primarily wants to smoke tobacco and not paper. Rather, there is a general desire to reduce the basis weight of the cigarette paper instead of increasing it. In addition, increasing the filler content impinges against limitations as the tensile strength of the paper is reduced too far and the paper is inclined to form dust during further processing. Further, with respect to the choice of the filler, there are legal and toxicological constraints. For example, titanium dioxide, particularly suitable for increasing whiteness and opacity, cannot be used in cigarette paper because of these limitations. The use of optical brighteners, although it might suggest itself technically, is excluded for cigarette papers for legal and toxicological reasons.